Field of the Invention
The present invention relates to an X-ray analysis device including an energy dispersive radiation detector, such as a superconducting transition edge sensor. The present application claims priority based on Japanese Patent Application No. 2015-166549 filed in Japan on Aug. 26, 2015, the disclosures of which are incorporated herein by reference in their entirety.
Description of Related Art
As an X-ray analysis device which can perform energy discrimination of X-rays, an energy dispersive X-ray detector (Energy Dispersive Spectroscopy, hereinafter, referred to as EDS) or a wavelength dispersive X-ray detector (Wavelength Dispersive Spectroscopy, hereinafter, referred to as WDS) is known.
An EDS is a type of X-ray detector which converts the energy of X-rays taken into the detector to an electrical signal in the detector and calculates energy according to the magnitude of the electrical signal. A WDS is a type of X-ray detector which monochromatizes (energy discrimination) X-rays using a spectroscope and detects the monochromatized X-rays using a proportional counter or the like.
As an EDS, a semiconductor detector, such as a SiLi (silicon lithium) type detector, a silicon drift type detector, or a germanium detector, is known. For example, a silicon lithium type or silicon drift type detector is frequently used in an element analysis device, such as an electron microscope, and can detect a wide range of energy of about 0.2 keV to 20 keV. However, since silicon is used in the detector, in principle, the properties of the detector depend on the bandgap (about 1.1 eV) of silicon, it is difficult to improve energy resolution to about 130 eV or more, and energy resolution is degraded by 10 times or more compared to a WDS.
In this way, when the energy resolution which is one index indicating the performance of an X-ray detector is, for example, 130 eV, this means that, if the X-ray detector is irradiated with X-rays, energy can be detected with uncertainty of about 130 eV. Accordingly, the smaller the uncertainty, the higher the energy resolution. That is, in a case of detecting characteristic X-rays having two adjacent spectrums, if the energy resolution becomes higher, the uncertainty becomes smaller. If the energy difference between two adjacent peaks is about 20 eV, the two peaks can be separated with energy resolution of about 20 eV to 30 eV in principle.
In recent years, superconducting X-ray detectors which are of the energy dispersive type and have energy resolution equivalent to a WDS have been attracting attention. Of superconducting X-ray detectors, a detector which has a superconducting transition edge sensor (Transition Edge Sensor, hereinafter, referred to as TES) is a high-sensitivity calorimeter using rapid change in resistance during superconduction-normal conduction transition of a metal thin film (for example, when change in temperature is several mK, change in resistance is 0.1Ω, or the like). A TES is also referred to as a microcalorimeter. Of the superconducting X-ray detectors, a superconducting tunnel junction (hereinafter, referred to as STJ) detector detects multiple electric charge carriers tunneling through an insulating layer of a Josephson element as signals. Of the superconducting X-ray detectors, a superconducting strip detector (for example, a Superconducting Single-Photon Detector, hereinafter, referred to as SSPD, or a Superconducting Strip-Line Detector, hereinafter, referred to as SSLD, or the like) is a detector using a fast relaxation process. Of the superconducting X-ray detectors, a microwave kinetic inductance detector (hereinafter, referred to as MKID) detects change in inductance.
The TES detects change in temperature in the TES occurring when fluorescent X-rays or characteristic X-rays generated from a sample due to irradiation of radiation, such as primary X-rays or primary electron beams, are incident, to analyze the sample. A TES has energy resolution higher than those of other detectors, and can obtain energy resolution of 10 eV or less, for example, with characteristic X-rays of 5.9 keV.
In a case where the TES is attached to a scanning electron microscope, a transmission electron microscope, or the like, characteristic X-rays which are generated from a sample irradiated with electron beams are acquired by the TES, whereby it is possible to easily separate a peak of an energy spectrum of characteristic X-rays (for example, Si-Kα, W-Mα, W-Mβ, and the like) which cannot be separated in a semiconductor type X-ray detector.
The counting efficiency of an X-ray detector is one index indicating the performance of the X-ray detector along with the energy resolution of the X-ray detector. The counting efficiency is an index which changes according to the area, thickness, and material of a radiation receiving portion of the X-ray detector, the distance between a radiation generation source and the X-ray detector, a maximum count rate of the X-ray detector, and the like. For example, the area of a radiation receiving portion of a general silicon drift type detector is several mm2 to hundreds of mm2, and the maximum count rate of a silicon drift type detector is tens of thousands of cps to hundreds of thousands of cps. The area of a radiation receiving portion of a TES is smaller than 1 mm2 in general, and the maximum count rate of a TES is hundreds of cps to thousands of cps.
In an energy dispersive X-ray detector, in general, the counting efficiency and the energy resolution are in a trade-off relationship. In a Si semiconductor detector, such as a silicon drift type detector, which of energy resolution or counting efficiency priority is given to can also be selected by switching a time constant of a counting circuit within the range of the capability of the detector. In order to realize high energy resolution, a signal from the X-ray detector needs to be extracted with high accuracy. To this end, a time constant of a filter or the time for extracting one signal is extended. As a result, the counting efficiency is inevitably lowered. In contrast, in order to raise the counting efficiency, a method which makes the time constant of the filter short or a method which increases the speed of data processing without effectively utilizing all pieces of information of detection signals is known; however, in these methods, the energy resolution is deteriorated. Furthermore, a method which uses a detection element designed for high counting efficiency and makes the area or thickness of the radiation receiving portion of the X-ray detector large is also known; however, energy resolution is sacrificed to some extent.
In mapping of a sample irradiated with charged particle beams, a microanalysis in a bulk sample, or the like, high counting efficiency is required. However, if a silicon drift type detector having energy resolution of about 100 eV to 200 eV, or the like is used for an unknown sample, in a case where there is an element close to energy of characteristic X-rays, the type of element cannot be discriminated, and even if the counting efficiency is high, there is a problem in that the accuracy of quantitative analysis is degraded.
In regard to such a problem, hitherto, a method which performs quantitative analysis using an analysis device with high counting efficiency based on a result of performing qualitative analysis using an analysis device with high energy resolution in advance has been known (see Japanese Unexamined Patent Application, First Publication No. 2002-71591). The analysis device with high energy resolution is an analysis device which uses a TES, an STJ, or the like capable of realizing extremely high resolution by means of a superconductive phenomenon. The analysis device with high counting efficiency is an analysis device which uses a silicon drift type detector or the like. In this method, a detector with high counting efficiency and a detector with high energy resolution are integrated or provided separately, and analysis is performed such that the features of the respective detectors are utilized.
Hitherto, a method which improves detection efficiency using a superconducting X-ray detector with high energy resolution has been known (see D. A. WOLLMAN, and five other, “High-resolution, energy-dispersive microcalorimeter spectrometer for X-ray microanalysis”, vol. 188, Pt 3, December 1997, pp. 196-223). In general, since a superconducting X-ray detector has a small detection area and low counting efficiency, in this method, X-rays are condensed on the detector using an optical element, whereby the small detection area is compensated for and the counting efficiency is improved.